The present invention generally relates to amplifier circuits and, more particularly, to an amplifier circuit of the type which uses a differential amplifier or an operational amplifier and is suitable for integration.
An operational amplifier generally involves a substantial level of noise due to the inherent procedure for the production and which may even amount to 3 nV/.sqroot.Hz to 20 nV/.sqroot.Hz. This results in a poor signal-to-noise or S/N ratio when the signal level is low. Meanwhile, the gain-to-bandwidth or GB product of an operational amplifier is generally small. A large GB product is unattainable unless the circuit construction is made disproportionately intricate and, therefore, expensive.
To settle these problems, there has been proposed a circuit which uses a differential amplifier at its initial input stage and couples an output of the differential amplifier to an operational amplifier. This prior art circuit achieves a noise voltage in terms of an input which is 3-4 nV/.sqroot.Hz, for example, which is a substantial improvement over an amplifier circuit which comprises an operational amplifier only. Another improvement is that the GB product is about 10 times the GB product of the amplifier circuit having only an operational amplifier.